1. Field of the Invention
The present invention relates to a shift device with a synchronizer for a transmission in which pressing force applied to a shift sleeve can be amplified into larger pressing force acting on a synchronizer ring while gears of the transmission are shifted, thereby reducing an operating force necessary for a driver or an actuator.
2. Description of the Related Art
A shift device with a synchronizer for a transmission of this kind is disclosed in Japanese Patent No. 4609796 and Japanese unexamined patent application publication No. 2007-285400.
The former conventional shift device has a plurality of thrust pieces. The thrust pieces are formed like a letter “H” when they are seen from an outer side in a radial direction, having a cross section like an arc along an inner surface of a shift sleeve. They are respectively arranged in axial-directional recesses of the shift sleeve at certain even intervals in a circumferential direction of the shift sleeve so as to move in an axial direction in cut-off portions of a hub. An upper surface of the thrust pieces are pushed by a spring shaped like a letter “C” outwardly in the radial direction on an inner surface of the shift sleeve.
The thrust pieces have a projection that is formed at a central position thereof. Each projection is capable of engaging with three circumferential grooves formed on central portions of predetermined three splines in lots of splines of the shift sleeve. The thrust pieces further have first slanted surfaces respectively formed on side surfaces of four projecting portions at four outer corners of the thrust pieces and second slanted surfaces respectively formed on outer surfaces of the projecting portion. The first slanted surfaces correspond to slanted surfaces on the cut-off portions of the hub, while the second slanted surfaces correspond to slanted surfaces formed on the recesses of the shift sleeve.
In the former conventional shift device, when the shift sleeve is positioned at a neutral position, the first slanted surfaces of the thrust pieces are free from a contact with the slanted surfaces of the hub, while the projections thereof engage with the circumferential grooves of the shift sleeve.
On the other hand, when the shift sleeve is shifted toward one of speed gears, the thrust pieces move towards the one of the speed gears in the axial direction in a state where bottom side surfaces of the thrust pieces press projections of a synchronizer ring. At this time, the slanted surfaces of the shift sleeve press the second slanted surfaces of the thrust pieces in the axial direction according to the value of force that presses the thrust pieces outwardly in the radial direction by the spring and the centrifugal force.
The thrust force causes a friction torque between friction surfaces of the synchronizer ring and a cone shaped portion integrally formed with the speed gear. The friction torque draws the synchronizer ring, thereby rotating the synchronizer ring at a predetermined angle in the circumferential direction relative to the hub. Accordingly, the projections of the synchronizer ring contact with the side surfaces of the thrust pieces to press them in the circumferential direction. Consequently, the thrust pieces swing in a state where radially outer surfaces of the thrust pieces are guided along the inner surface of the shift sleeve. This swing movement causes the first slanted surfaces thereof to contact with the slanted surfaces of the hub. The thrust pieces change the friction torque into the thrust pressing the synchronizer ring through the first slanted surfaces of the thrust pieces and the slanted surfaces of the hub.
That is, the bottom side surfaces of the thrust pieces press the projections of the synchronizer ring in the axial direction. When the shift sleeve further moves toward the speed gear, chamfers of the splines of the shift sleeves contact with chamfers of the synchronizer ring. At this time, the thrust pieces are forced to move inwardly in the radial direction against elastic force of the spring through the slanted surfaces of the shift sleeve. This state is maintained until the synchronization is ended, so that the shift sleeve is prevented from further advancing toward the speed gear. In the synchronization operation, the friction torque is determined due to force generated at the synchronizer ring being pressed by the shift sleeve and force generated between the first slanted surfaces of the thrust pieces and the slanted surface of the hub, thereby being amplified. When the synchronization ends, the shift sleeve rotates the synchronizer ring back to its original position, and then the shift sleeve further moves in the axial direction to engage with splines of the speed gear.
On the other hand, the latter conventional shift device has a plurality of thrust pieces similar to those of the former conventional shift device. It differs from the former conventional shift device mainly in the construction and function of springs.
That is, three springs are respectively provided in the thrust pieces arranged at even intervals in the circumferential direction. The springs in the latter conventional shift device use three coil spring instead of only one spring shaped like C in the former conventional shift device. Each coil spring is capable of pressing a ball against a radially inner surface of the shift sleeve, but it is not capable of pressing the thrust pieces outwardly and/or inwardly in the radial direction. The balls and the springs are contained in holders that are inserted in holes formed in center portions of the thrust pieces, respectively. The holders are not fixed to the thrust pieces, and bottom portions (the most-inner portion in the radial direction) thereof contact with bottom surfaces of cut-off portions of a hub so that the springs can press the balls against the inner surface of the shift sleeve and the bottom surfaces of the hub receive reaction forces of the springs.
Between the holders and the thrust pieces, stoppers are provided so as to prevent them from being separated from each other. The operation of the latter conventional shift device is similar to the former conventional shift device except that, in the latter conventional shift device, the springs do not press the thrust pieces on the inner surface of the shift sleeve, but the thrust pieces are pressed on the inner surface of the shift sleeve by the centrifugal force acting on the thrust pieces when a shaft is rotating.
These former and latter conventional shift devices with the synchronizers, however, encounter the following problem.
In the both shift device, the thrust pieces make a swing movement in the amplification operation, where the axes of the thrust pieces and an axis of the shift sleeve shift from in a co-axial relationship to in an oblique relationship. As the both surfaces are formed as a partial portion of an outer surface of a circular cylinder, their axes of the thrust pieces deviate from the axis of the shift sleeve, inclining to the hub when the thrust pieces swing in the synchronizer operation. This swing and inclination movement of the thrust pieces generates a resistance between the radially outer surfaces of the thrust pieces and the radially inner surfaces of the shift sleeve because the radially outer surfaces of the thrust pieces are pressed on the radially inner surface of the shift sleeve by external force such as elastic force of the spring and the centrifugal force. The resistance deteriorates the amplification performance in the synchronization operation of the shift device.
It is, therefore, an object of the present invention is to provide a shift device with a synchronizer for a transmission which overcomes the foregoing drawbacks and in which can decrease a resistance generated due to a contact between thrust pieces and a shift sleeve when the thrust pieces swing relative to a hub in a synchronization operation, thereby improving a synchronization performance.